Influence Of Operating Condition For Mixing In Liquid-Solid Stirred Tank

Stirred tank is a common mixing equipment.In an stirred tank,mixed substance moves with the stirred paddle,acting with the fixed baffle,and forms complicated quasi-periodicity three-dimensional turbulent.In the present,Research on liquid-solid mixing has two methods:experimental measurement and numerical simulation. Experimental measurement is to measure liquid-solid flowing using several kinds of measurement devices through setting up experimental tables,numerical simulation is by setting up a mathematic model which describes the flow field in stirred tank,and undoes the Equations so as to simulate the flow field.However,the experimental table is limited by experimental technique and devices,such as:as for the tracking the high concentrated particle's pathway and observing the particle's moving speed,the conventional optical measurement will result in inactivation.By setting up a mathematical equation about the particle's or quasi-liquor's pathway and using the computer to undo the particle's pathway,the numerical simulation can obtain more accurately the flow field's complexor, speed,particle distribution,and so on.For example,for different concentrated operating modes,just adopting different theoretical models can do the simulations very well.This thesis is adopting the liquid-solid agitated mixed system in the stirred tank with a flat turbine as the subject investigated.From the point of numerical simulation,it does the research about the influence on the particle suspension's quality impacted by liquid-solid mixed process and operational conditions in the stirred tank.This thesis is divided into three main parts.1.This thesis uses the particle dynamics theory,by setting up particle's compression stress and shearing strength to reflect the influence about particle's movement by particle's crash,and sets up Euler-Euler model which is applicable for liquid-solid turbulent:particles are assumed as quasi-liquor.It focuses on the two sets of equations about quasi-liquor and liquid and then connects the relational expression of the hydromechanics parameters(such as the state equation) to form a closed equations set in order to obtain the internal flow field information by undoing the equations set to get the quasi-liquor and liquid flowing movement.It uses this model to simulate the liquid-solid three-dimensional turbulent in the stirred tank,and makes a contrast with the experimental results.The diameter of the particles selected in this experiment is: d_p =264μm,the mediator water's density is:p_l 1000kg/m~3,and the viscosity is:μ= 0.001kg/m.s.The experiment is taken in the room temperature,the solid content (capacity's density ) is 10%,and the speed of rotation of the stirred paddle is 20rps.By comparing the capacity's density of the sample,it is found that the simulative result and the experimental result are consistent with each other very well.It indicates that Euler-Euler model of this thesis is able to reflect the liquid-solid turbulent flowing characteristics in the stirred tank,the numerical simulation can anticipate the fluid field distribution and some relevant research.2.This thesis is using the method of numerical simulation to present the more effective particle suspension's judgment criterion.Particle suspension degree includes incomplete suspension,off-bottom suspension and uniform suspension.Some industrial process demands the particle suspend uniformly in the stirred tank,that is to say,every particle's capacity density in the stirred tank is nearly the same.For this kind of liquid-solid flowing system,it is essential to find out the judgment criterion of the uniformed suspension of particles.The two judgment criterions of solid suspension research are:setting up the average density of the sample at the bottom of the stirred tank and setting up the average density of the sample at the 90%height of the stirred tank. When the selected surface is reaching the uniform density,it is believed that the solid in the stirred tank is suspending uniformly.This thesis is doing the simulation using the density of 8%,10%and 15%liquid-solid mixed systems,sets up the average density in the selected surface at the bottom and at the 90%height,and builds eight selected points to monitor the average density in the stirred tank.When the selected surface is reaching the average density,then the solid in the stirred tank is reaching the uniform suspension. Calculating the two density standard deviation of the two kinds of judgment criteria can compare the solid uniform suspension degree.Therefore,we can tell the advantages and disadvantages of the two judgment criteria.The simulative experimental results indicate that the two kinds of judgment criteria are both applicable for the high density liquid-solid mixed system.The differences of the two solid suspension degree are little. With the increase of the density,the gap is becoming smaller.It is a little better to adopt the bottom selection judgment.According this,adopting the bottom selection judgment can fulfill the requirements of simple operation,fair accuracy and wide application in the industrial application.This becomes the basis of the following research and an innovation of this thesis.3.The influence of liquid-solid mixing process in the stirred tank by operating conditions.The operating conditions include the rotating speed of stirred paddle,particle density and particle diameter.Based on the mathematical model and solid suspension judgment,this thesis studies the influence of the mixing process by operating conditions through the simulative liquid-solid mixing process in the stirred tank.1) The thesis studies the influence by ten kinds of rotate speeds of stirred paddles N=2.5～40rps and observes the solid suspension quality and mixng time under different rotating speeds.The conclusion is that solid suspension quality is increasing as the speed increasing.However,when the speed is higher than N=16.1rps,the influence is becoming smaller;the mixing time is decreasing as the speed increasing,and the decay degree is becoming smaller.2) This thesis studies the influence of the speed and suspension degree by different particle densities:8%,10%and 15%.Through the observation of the combined speed, axial speed,radial speed and tangential speed's change along with the radial speed,it is known that the increase of the particle density will bring in the dramatic decrease of the axial speed and combined speed,a small decrease of the tangential speed and no change of the radial speed.Through the observation of the three density distribution of the three axial lines:R/T=0.4,0.6,0.9,it is known that solid suspension quality is becoming smaller as the particle density increasing.3) This thesis studies the influence of solid speed and suspension degree by particle diameters of three sizes:0.264mm,0.400mm,0.750mm.Through the observation of the changes of the combined speed,axial speed,radial speed and tangential speed,it is known that the increase of the particle's diameter will bring in the dramatic decrease of axial speed and combined speed,a small decrease of tangential speed and no change of the radial speed.Through the observation of the density distribution of the three axial lines:R/T=0.4,0.6,0.9,we know that solid suspension quality is decreasing as the particle's diameter is increasing.Through the observation of the density distribution of the three radial lines:z=30,120,240,it is known that there is no remarkable change of the radial density as the increase of particle's diameter.There are five chapters in this thesis.Chapter One introduces the research background and the present situation of the liquid-solid mix study,and gives a brief introduction of the research contents and methods in this thesis.Chapter Two introduces the stirred tank briefly,What's more,the discussion of the liquid-solid system includes the aim of liquid-solid mix.finally the research methods,simulative theory and the software under use are introduced.Chapter Three mainly includes the introduction of the computer simulation of this thesis,which includes the distribution methods of simulative calculation thought,simulative theoretical model,physical model-setting process and physical graticle,and parameter setting of the software and the setting up of the parallel calculation platform.Chapter Four includes the examination of the simulative model,the presentation of the judgment criteria of the solid suspension and the influence of the mixed process by the operational conditions.Chapter Five is the conclusion part of the thesis which includes the achievements obtained,the deficiency of this thesis and the anticipation of the future research.